For reflection film or coating used on optical recording media such as CD (Compact Disc), DVD (Digital Versatile Disc), and the like or photo-reflective conductive coating used on reflection type STN (Super Twist Nematic) liquid crystal display devices, organic EL (Electroluminescence) display devices and the like, generally aluminum (Al) or Al alloys are used.
Such photoreflective thin film put to usages with those optical recording media, liquid crystal display devices, organic EL display devices and the like is generally prepared by making sputtering target material having desired properties, and film-forming using the sputtering target material by such methods as RF (radiofrequency) sputtering or DC (direct current) sputtering.
Thin films formed of Al or Al alloys which are prepared by such methods exhibit moderate degrees of reflectance, low electrical resistance and furthermore, stable corrosion resistance even in the air because passive state films are formed in the surface layers. However, reflectance of thin film made of Al or an Al alloy is, for example, around 80% of light having a wavelength of 700 nm, which is not fully satisfactory for the usages requiring high reflectance.
It has been hence proposed to form thin films using gold (Au) or silver (Ag) as sputtering target material instead of Al or Al alloys, for e.g., optical disc media represented by CD or DVD which require high reflectance. Also for reflection type STN liquid crystal display devices, use of Ag having high reflectance as the material for the thin film has been proposed.
Optical disc media represented by CD or DVD, reflection type STN liquid crystal display devices and the like are liable to be exposed to high temperatures under the conditions of their use. Where Ag is used, there arises a problem that the film may cause aggregation under high temperatures, e.g., 200° C. or higher, to decrease the reflectance.